A conventional intracranial electrode is used for measuring an intracranial brain signal directly, examining a brain function by applying electrical stimulation to the brain, and treating intractable pain or involuntary movement. A typical intracranial electrode can be roughly classified into (1) a deep brain stimulation electrode which is inserted deep into the brain and (2) a subdural electrode which is placed on the brain surface.
In the deep brain stimulation electrode, generally, electrodes are arranged at even intervals on a thin tube. Such a deep brain stimulation electrode is used for measuring brain signals and electrical stimulation in a deep part of the brain. On the other hand, in the subdural electrode, generally, electrodes made of such as platinum are fixed on one side of an electrode sheet formed from such as silicone. Such a subdural electrode is used for measuring a brain signal and electrical stimulation on the brain surface.
More specifically, a deep brain stimulation electrode includes such as a lead wire type electrode which is used in deep brain stimulation for a parkinsonian patient. Moreover, a subdural electrode includes such as an intracranially implantable diagnostic electrode in which an electrode is held in intermembrane space of a silicone double membrane whose thickness is 1 mm or less (e.g. see Patent Literature 1).
Moreover, a double-sided electrode is commercially available as another intercranial electrode. In the double-sided electrode, electrodes are provided on both sides of a silicone sheet. The double-sided electrode is used for being inserted into the interhemispheric fissure (section between right and left cerebral hemispheres), and measuring brain signals from right and left cerebral hemispheres.
Moreover, a conventional electrode, which includes a structure having a plurality of electrodes therein and arranged along a tissue surface of such as a heart, has been used (e.g. see Patent Literature 2). Note that, the structure is produced by spraying a material on a surface of a mold having a shape of a target tissue. Further, positions of the electrodes in the structure are predetermined on the mold.
The cerebral cortex can be divided into two regions. One region is the brain surface called “gyri”, and the other region is a recess in the brain surface called “sulci”. The sulci have an area three times larger than the gyri. Therefore, the sulci are considered to serve an important role for a brain function.
In these years, it is experimentally proved that, a brain signal in the sulcus has a drastically different characteristic from a brain signal in the gyrus in some cases (e.g. see Non Patent Literatures 1 and 3). Especially, it is proved that, depending on kinds of motion, brain signal characteristics are drastically different from one another in the sulcus and the gyrus in the primary motor area of the cerebral cortex.
Citation List
Patent Literature 1
U.S. Pat. No. 5,044,368 A (Publication Date: Sep. 9, 1991)
Patent Literature 2
U.S. Pat. No. 5,961,909 A (Publication Date: Oct. 5, 1999)
Non Patent Literature 1
M. Hirata et al., Hierarchical network in the human sensorimotor cortex: coherence analysis using supragyral and intrasulcal surface electrodes, in Abstracts of The 29th Annual Meeting of the Japan Neuroscience Society (2006)
Non Patent Literature 2
Y. Saitoh et al., Primary motor cortex stimulation within the central sulcus for treating deafferentation pain, Acta Neurochir [Suppl.]87:149-152, (2003)
Non Patent Literature 3
T. Yanagisawa et al., Neural decoding using gyral and intrasulcal electrocorticograms. Neuroimage (in press)
As mentioned above, in these years, importance of the sulcus for a brain function has been understood. In order to analyze a more detailed role of the sulcus for a brain function, it is necessary to measure an intersulcus brain signal, and to locally apply electrical stimulation to the sulcus. For example, Non Patent Literature 2 recites that, for treating intractable pain, the motor area within the central sulcus is stimulated by an electrode arranged in the sulcus.
However, at present, measurement of a brain signal and electrical stimulation in the sulcus are rarely performed. Moreover, there is no effective electrode for the measurement and the stimulation. For example, electrodes recited in Patent Literature 1 and Non Patent Literature 2 have planar shapes, and besides, the electrodes are hard. Therefore, if the electrode is placed in the sulcus, a cerebral dysfunction would possibly occur due to pressure applied by the electrode. Moreover, even in a case where the electrode is placed on a surface of the brain, the electrode sheet would possibly apply pressure on the brain when the electrode (sheet) is large. Further, an electrode placed on the brain surface has an advantage that spatial resolution can be improved because the electrode measures a brain signal from the brain surface. However, in the conventional electrodes, an interval between electrodes is approximately 1 cm in general, and it is hard to regard their spatial resolution as high enough.
Moreover, a conventional method for producing an electrode has a problem in that the method cannot produce a thin electrode having a uniform thickness. For example, the method in Patent Literature 2 has a problem in that a structure is formed by simply spraying a material on a mold, whereby a thickness of the structure cannot be even. Further, the method in Patent Literature 2 also has a problem in that a structure is formed after positions of electrodes are predetermined, and this leads to low positional flexibility of the electrodes.